The ever increasing demand for energy together with the desire to protect the environment from pollutants has resulted in considerable recent study of upgrading coal by removing sulfur therefrom. Coal as a fuel is an abundant source of energy comprising mostly carbon and small percentages of hydrogen, sulfur, and ash. When coal is burned to produce energy the presence of the sulfur and ash is generally undesirable. The sulfur and ash enter the atmosphere with the combustion products creating air pollution. While most of the ash can be prevented from entering the atmosphere little can be done with the sulfur. Hence, there is a desire for coals with a lower sulfur content. As a rule of thumb, coals are unacceptable if they contain more than about one percent sulfur. Unfortunately, many deposits of coal contain more than one percent sulfur usually present in the forms of pyrite, free sulfur and organic sulfur.
Numerous processes have been proposed for upgrading coal by sulfur removal. A portion of the sulfur can be removed by existing technology, namely, "coal washing." This process removes pyritic sulfur which exists in natural coal in the form of separate particles ranging from the very small to large nuggets. Approximately 30 to 80 percent of pyritic sulfur can be removed by washing, but usually this is not sufficient. The sulfur which cannot be removed by washing can only be removed by chemical processes. Generally speaking, there are three chemical coal desulfurization techniques disclosed in the prior art: (1) oxidation of the sulfur in the coal to form soluble sulfates; (2) reduction of the sulfur to elemental sulfur in which form it can be vaporized or removed by organic solvents; and (3) reaction with hydrogen to form gaseous hydrogen sulfide. Some processes are a combination of two of the above and some simply do not fit any of these classifications.
For any sulfur removal process to be practical it must be adaptable to the handling of large volumes of materials very economically. Hence, the reactants should be inexpensive and capable of being recycled and the apparatus should be rugged and simple.